Water-soluble polymers have excellent functional performances as thickeners, cohesive agents, flocculants, desiccants, drying agents, surface modifiers, cohesive improvers, dispersants, and the like, and have been frequently used industrially. Such water-soluble polymers have been variously used in, for example, medical drugs, coating materials, paper manufacture, cleaning agents and cosmetics, water treatment, fiber treatment, civil engineering and construction, art of agriculture, gardening, adhesives, ceramic industry, manufacturing process, and other fields as well as excavated soil treatment agents, additives for anti-inflammatory analgesic plasters or cataplasms, and dredged soil treatment agents. Water-soluble polymers with high quality in viscosity or residual amount of monomers have been used for foods or animal feeds as a thickener or an accelerator for ravelling, for example.
As conventional production methods of water-soluble polymers, disclosed is a process for producing sodium acrylate with a high concentration by a batch polymerization under control of foaming arising or temperature, wherein a batch type reaction apparatus with a cooling device, a tank reactor, a stirring mixer is disclosed (for example, referring to Japanese Kokai Publication No. 2004-244617 (pages 2, 8 to 10, and 41). However, this production method needs a large apparatus for securing high productivity, and has room for improvement in order to produce a polymer having a desired molecular weight distribution because the obtained polymer has a relatively wide molecular weight distribution (Mw/Mn). The molecular weight distribution (Mw/Mn) is calculated by dividing a weight average molecular weight (Mw) by a number average molecular weight (Mn).
Disclosed is a method for continuously producing polyacrylic acid with a narrow molecular weight distribution in which a plurality of reaction apparatuses are serially disposed (for example, referring to Japanese Kokai Publication No. 2003-002909, pages 2, 4 to 7, and Japanese Kokai Publication No. 2003-040912, pages 2, 5 to 8). And in Examples, disclosed is a continuous stirred tank serial reaction apparatus composed of first to third tank reactors, a cooling device in the second reactor if needed, and a stirring mixer. This reaction apparatus is a continuous apparatus, but the stirred tanks are serially coupled as the first to third reactors. A reaction liquid is made to flow from the first reactor to the third reactor in this order and discharged from the third reactor. Such serial reaction apparatuses have room for improvement in reduction of reaction time, productivity, or heat removal efficiency in order to industrially efficiently produce polyacrylic acid.
In addition, disclosed is a method for continuously producing polyacrylic acid with a narrow molecular weight distribution using a loop-type circulation line (for example, referring to Japanese Kokai Publication No. 2001-098002, pages 2 to 9), wherein a circulating type continuous reaction apparatus having a piping reactor and a motionless mixer is disclosed. However, this production method has room for improvement in order to further improve the productivity by sufficiently improving the heat removal efficiency and reducing the reaction time.